The embodiments disclosed herein generally relate to optical sensors and more particularly to an optical sensor calibration system and a corresponding method for calibrating optical sensors.
Spectrophotometers are used to make color measurements in printing applications. In order for a particular spectrophotometer unit to produce precise color measurements, the unit is calibrated by calculating a number of reflectivities with respect to known reference reflectance values. Usually, these reference values are provided by measuring a white or nearly white standard reference surface that has known reflectivities at the wavelengths of interest.
Two factors are vital to the accuracy of the spectrophotometer's reflectivity calculations. First, the spectrophotometer's measurement of the reference surface must represent the current state of the instrument. This may vary with component age, temperature shifts, or optical contamination. A spectrophotometer is usually equipped with some internal calibration routine or other method to compensate for these changes in system response. The instrument's manufacturer will generally specify the conditions that warrant this occasional recalibration. The second important factor is accurate knowledge of the actual reflectance of the standard reference surface at the precise wavelengths of interest to the spectrophotometer. These values are stored inside the spectrophotometer prior to use. Ideally, the reference surface would be 100% reflective across the spectrum of interest (the visible light range for color spectrophotometers), thus making the need for device specific knowledge of reference surface properties unnecessary. However, this would be expensive and is not really necessary in practice. Another possible scenario would be to use reference surfaces that have identical reflectance properties from one unit to the next, thus allowing interchangeability, but in general this would not be practical.
The current technique is that manufacturers characterize and serialize each individual reference surface. The vendor provides the buyer with an exact specification of reflectance and color of the reference material. The spectrophotometer and reference surface are shipped as “siblings” and must remain together. Sometimes at the time of manufacture the reflectance data for the reference surface is pre-loaded into non-volatile memory (NVM) on the spectrophotometer. Often, manufacturers publish reflectance data for an individual reference surface in printed form so that it can be manually loaded into the spectrophotometer. This might be necessary due to memory failure in the spectrophotometer, or if the reference surface requires replacement due to loss or damage. However, this practice presents logistical difficulties and introduces the potential for human error. Furthermore, in order to maintain color measurement accuracy over the life of the instrument, a spectrophotometer periodically is recalibrated using the reference surface. Recalibration may be difficult or impossible if the reference surface is lost or damaged.
U.S. Pat. No. 5,267,178 describes a spectrophotometer equipped with a serial interface to which a bar code reader can be connected. Using the bar code reader, calibration and configuration data or functional commands for the spectrophotometer can be read from the bar code and transmitted to the computer in the spectrophotometer. U.S. Pat. No. 5,267,178 also mentions storing the data on another type of data carrier such as one that can be read by a conventional reading device, such as a magnetic tape reader or a diskette drive, instead of a bar code reader.
Various disclosures are available involving the calibration of sensors that are affiliated with printing devices. Commonly assigned U.S. Pat. No. 6,972,867 involves in-line image quality testing of a printer. Commonly assigned U.S. Pat. No. 6,567,188 describes a calibration system for an input scanner for a copier. U.S. Pat. No. 6,035,152, also commonly assigned, describes calibration of a printing machine by measuring test patches on an imaging surface.
It would be useful to develop a method by which relevant parameters would be stored and communicated automatically between a reference component and an optical sensor, thus eliminating the need for human intervention, facilitating hardware interchangeability and eliminating unique dependencies between the reference component and the optical sensor.